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I am using the sportstracklive app on my EVO to track my fitness activities. One issues is that elevation profiles are incorrect. For example, a riverfront section of a recent bike ride was recorded on the elevation profile as 101 feet below sea level.

Is there a way to calibrate the gps so that I get more accurate results?

Also, I'm pretty sure that the app itself is not at fault. I've experimented with a couple of other fitness tracking apps that are producing similar results; so that's why I'm thinking that the issue is with the gps.

Here're a couple of screen grabs of the route. The one with the red shading is correct and was generated by ridewithgps. The one with blue shading is generated by sportstracklive on the EVO gps (blue shaded area is elevation profile). A profile from my Garmin looks like the correct (red) one.

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How do you know it was wrong? Many low-lying areas (like valleys with rivers in them) are below sea level. –  Matthew Read Jan 5 '11 at 22:54
    
Well, 101 feet is pretty far down there. Death Valley is less than 300 feet below sea level. –  Al E. Jan 5 '11 at 23:02
    
I know the elevation of my location by other means. According to USGS topographic maps and Google Earth, the area recorded as -101 feet is actually 35 feet. The river surface is 10 feet and the channel is 45 feet deep. So, even if the gps thought I was on the river bottom it was still incorrect. –  wdypdx22 Jan 5 '11 at 23:03
    
@wdypdx22: Does it matter that much? Isn't the difference in elevation the key datum? –  Al E. Jan 5 '11 at 23:03
    
...And my hope is that when I am in unknown areas on hikes and bike rides, I can get relatively accurate profiles. I also have a Garmin GPS device which correlates closely with topo maps. –  wdypdx22 Jan 5 '11 at 23:08
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2 Answers

up vote 7 down vote accepted

Your GPS is nothing more than a precise clock, passively receiving signals. There is, unfortunately, nothing to calibrate. Source

If you'd like, consider going to the problem locations with an app such as "GPS Test" to view the internals of your phone's GPS to try and understand the problem, although there isn't much you can do about it.

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Bummer. The EVO must have a crappy gps implementation. My Garmin generated profiles overlay very tightly with the routes I create on ridewithgps. Was hoping to not have to carry it along...it's an older model and a bit heavy, but accurate... –  wdypdx22 Jan 6 '11 at 0:27
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Guess I was confused. The Garmin has a barometric altimeter, so long as I calibrate, I get accurate elevation profiles. I had forgotten that I wasn't making a gps calibration. –  wdypdx22 Jan 6 '11 at 1:03
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You can use the GPS Status app to tell you exactly what your device's GPS is doing. It will tell you things like your current GPS calculated position and the accuracy of that position (in the Error field).

It may be that at times your device's GPS is not getting a good lock, maybe one that's only accurate to 30m (which is what mine's reporting curretly in a built up area, and would explain a 100ft altitude error) and the apps you're using aren't taking into account the accuracy of the current fix. Also if you're out exercising in the countryside, your phone won't be able to take advantage of the closely packed cell towers and nearby wifi points that it uses to assist the GPS in urban areas.

You should also be aware that the altitude accuracy of a GPS unit is normally much less accurate than the horizontal accuracy, this is a problem inherent in GPS, particularly with the cheaper GPS chips used in phones as opposed to those used in dedicated GPS units like your Garmin, see GPS Altitude Readout > How Accurate?.

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Thanks for the GPS Altitude Readout link. Explains a lot. After re-checking today, I find that the EVO gps is quite accurate horizontally. And in a comparison with known altitudes, it's off roughly by 80' to 110'. –  wdypdx22 Jan 6 '11 at 23:13
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